refactor: DDD/CQRS architecture, unified crate layout

- crates: common→application, api→presentation, infrastructure/*→adapters/*
- new crates: api-types, infra-wiring
- domain: errors/, models/, value_objects/, ports/, services/
- application: CQRS use cases (songs/, tabs/) w/ commands, queries, deps
- unified DomainError replaces RepositoryError
- workspace deps, unused dep cleanup
- fix: parse plain-text chord lines (UG drops spans mid-song)
- tests extracted to separate modules (tests/ dirs)

crates/application/src/songs/get_song.rs

@@ -0,0 +1,12 @@
+use domain::errors::DomainError;
+use domain::models::Song;
+
+use super::deps::SongQueryDeps;
+use super::queries::GetSongQuery;
+
+pub async fn execute(
+    deps: &SongQueryDeps,
+    query: GetSongQuery,
+) -> Result<Option<Song>, DomainError> {
+    deps.repo.get(query.id).await
+}